North Cascades Geology

The Work of Gravity

Erosion is ultimately an effect of gravity, that force which tends to keep the Earth a spheroid in spite of internal upheavals. Running water, ice, and hikers cutting switchbacks are merely intermediaries, helping move material downhill. In the mountains, gravity also works directly. Hillsides are moving slowly downhill most of the time in a process called creep. If a mass of material breaks away to fall downhill more rapidly it is a landslide or rockfall. Creep and landslides eventually turn the steep-walled gorges sawed by rivers into V-shaped valleys and hide the U-shape of glacial channels.

Monstrous Church Mountain landslide covering the floor of the Nooksack North Fork valley, as viewed from Skyline Divide.

In steep-walled, glacier-carved valleys, landslides can be major actors in the erosional scene. Some move slowly, creeping downhill at a few inches or yards per year. Others are catastrophic. An especially large catastrophic landslide filled the valley bottom near Glacier on the North Fork of the Nooksack River about 2,400 years ago. Another large catastrophic landslide on the West Fork of the Pasayten River has not been dated directly but may have come down onto the last glacier to occupy the valley about 14,000 years ago. A landslide dammed the Skagit River north of Marblemount for about 1,500 years, long enough for a considerable amount of sediment to accumulate in the lake that formed. Radiocarbon ages of drowned trees in the lake indicate the mountainside collapsed about 8,400 years ago. Geologists do not know for sure what triggered these particularly large slides, but they suspect that earthquakes may have been the cause.

The smoother and lower the hills get, the slower the descent of rock and debris to the rivers and the slower the removal of the material to the sea. But gravity never tires and, given enough time, it will smooth even the boldest peaks of the North Cascades into low, rolling hills.

Material in this site has been adapted from a book,
Geology of the North Cascades: A Mountain Mosaic by R. Tabor
and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published
by The Mountaineers, Seattle.